A common type of adjustable chair is the motorized wheel chair, which has helped make the life of a handicapped person more comfortable and more independent. Motorized wheel chairs generally have a seat frame and a back frame, both of which are adjustable relative to a base frame. A back support is usually provided on the back frame to receive the trunk of the user. In some cases, the back support is movable relative to the back frame. The adjustment of these components is made possible by what is referred to in the art as "tilt", "recline" and "zero-shear" mechanisms. Several terms relating to these mechanisms and used commonly amongst those skilled in the art are as follows:
"Tilt" refers to a change in angle of the seat frame relative to the wheel chair frame, while the angle of the back frame relative to the seat frame stays constant.
"Recline" refers to a change in the angle of the back frame relative to the seat frame. In this case, the angle of the back frame relative to the seat frame increases or decreases to the desired back frame position while the seat frame angle relative to the wheel chair frame stays constant.
"Shearing" refers to the shear or tangential forces that occur as a result of the relative displacement between the user's trunk and the back support. This occurs because the center of rotation of the user's trunk (approximately located at the user's hip joint) does not coincide with the axis of rotation of the back frame. Shearing is a problem because it can cause decubitus ulcers (pressure sores) on the user's body and because it creates problems in maintaining the correct position of the postural supports (e.g. chest pads, headrests) and control devices (e.g. chin control systems) relative to the user.
"Zero-Shear" is an industry used term that refers to a reclining back system that uses a mechanism which significantly reduces (but not necessarily completely eliminates) the effects of shear. Because the back support moves with the user, postural support and control devices are often attached to the back support to maintain correct positioning of these devices relative to the user.
Typically, zero-shear systems utilize a sliding back support that is either attached to the back frame with glide blocks or rollers. Sliding back supports are usually actuated with mechanical linkages, cam or cable systems and travel at a fixed speed relative to the rotation of the back frame. The conventional devices have been found to be unsatisfactory, since they fail to take into account the specific needs of each user, which tend to change from one user to another.
Conventional motorized wheel chairs have also been outfitted with "anti-tipping" wheels emerging from the chair to support the chair from overturning. However, there are some instances where conventional "anti-tipping" wheels fail to prevent overturning because they fail to take into account that the center of gravity of the user may shift as the chair is adjusted.
It is therefore an object of the present invention to obviate or mitigate the above-mentioned disadvantages.